1. Field of the Invention
The invention relates to a shift control technology for an automatic transmission whose shifting is controlled based on a shift map that is set in advance, and more particularly, to a shift control technology for an automatic transmission in a highland area.
2. Description of the Related Art
In general, a control apparatus for an automatic transmission that is installed in a vehicle determines a shift speed based on two values, that are, a vehicle speed and a throttle opening amount, according to a shift map which is composed of plural upshift lines and downshift lines that are set in advance using the vehicle speed and the throttle opening amount (engine load) as parameters, and automatically changes a shift speed.
Examples of the shift map include a shift map for running in a flat road (hereinafter, referred to as “flat road shift map” or “normal running shift map”) which allows the shift speed to be appropriately changed, a shift map for running on a descending slope road (hereinafter, referred to as “descending slope road shift map”) in which a shift speed change line is set so as to be on a higher vehicle speed side in a small throttle opening amount region such that a low shift speed region is set to be large as compared with the normal running shift map, and a shift map for running on an ascending slope road (hereinafter, referred to as “ascending slope road shift map”) in which a specific shift speed region is set to be large as compared with the normal running shift map. The control apparatus for an automatic transmission detects an inclination of a road surface on which a vehicle is running. When the control apparatus determines that the vehicle is running on a descending slope road based on the result of detection, switches the shift map from the normal running shift map to the descending slope road shift map, and performs shift control based on the descending slope road shift map. Thus, engine brake that is suitable for running on the descending slope road can be applied. Alternatively, when the control apparatus determines that the vehicle is running on an ascending slope road based on the result of detection, the control apparatus switches the shift map from the normal running shift map to the ascending slope road shift map, and performs shift control based on the ascending slope road shift map. Thus, torque that is suitable for running on the ascending slope road can be obtained.
In such a case, in the ascending slope road shift map, a specific shift speed region is set to be large as compared with the normal running shift map. For example, in the case of a vehicle having four shift speeds from first shift speed to fourth shift speed, an upshift line from third speed to fourth speed (hereinafter referred to as “third to fourth upshift line”) or the like is moved to a higher vehicle speed side, and a third to second downshift line is moved to a larger throttle opening amount side such that a third speed region becomes larger. Since a driver requires large torque when running on an ascending slope road, the driver does not want to perform upshifting from third speed to fourth speed in most cases, even when a predetermined vehicle speed is achieved, unlike the case of normal running. Therefore, the ascending slope road shift map is set such that upshifting becomes less likely to be performed as compared with the normal running shift map. Also, when running on an ascending slope road, the throttle opening amount becomes relatively large as compared with when running on a flat road, the combination of the vehicle speed and the throttle opening amount is often positioned in the vicinity of the third to second downshift line in the normal running shift map, and downshifting that has not been expected by the driver is performed, which makes the driver uncomfortable. Therefore, the ascending slope road shift map is set such that downshifting becomes less likely to be performed as compared with the normal running shift map.
Meanwhile, in a highland area whose altitude is relatively high, atmospheric pressure is relatively low and an amount of oxygen supplied to an engine is small as compared with a lowland area, and therefore the output of the engine decreases. Accordingly, when running in such a highland area, the throttle opening amount becomes relatively large as compared with when running on a lowland area. In such a case, when running on a flat road in a highland area, shift control may be performed based on the flat road shift map.
However, when running on an ascending slope road in a highland area, shift control is performed based on shift characteristics for running on an ascending slope road in which the third to second downshift line is moved to a larger throttle opening amount side. In the case where shift control is performed based on the shift characteristics, even when the throttle opening amount becomes relatively large, downshifting is less likely to be performed, and the vehicle cannot run forcefully at second shift speed though the driver wants to do so.
Japanese Patent Laid-Open Publication No. 2000-88091 discloses a control apparatus for an automatic transmission which controls downshifting from a specific shift speed according to the altitude of an ascending slope road on which a vehicle is running. The control apparatus performs control according to shift characteristics in which a value concerning a vehicle speed and a value concerning engine load are used as parameters. Examples of the shift characteristics include shift characteristics for running on a flat road, and shift characteristics for running on an ascending slope road in which a predetermined downshift line is moved to a higher engine load side. The control apparatus includes an inclination detection sensor which detects the inclination of a running road, and a shift characteristics switch circuit which switches the shift characteristics from the shift characteristics for running on a flat road to the shift characteristics for running on an ascending slope road when the inclination of the running road that is detected by the inclination detection sensor is equal to or larger than a predetermined inclination. The control apparatus further includes an atmospheric pressure detection sensor which detects atmospheric pressure, and a shift characteristics change circuit which changes a predetermined downshift line in the shift characteristics to a lower engine load side when the atmospheric pressure detected by the atmospheric pressure detection sensor is low, as compared with when the atmospheric pressure is high in the case where the shift characteristics switch circuit selects the shift characteristics for running on an ascending slope road.
In the control apparatus, in the case where the shift characteristics for running on an ascending slope road is selected, the predetermined downshift line is changed to a lower engine load side when the atmospheric pressure detected by the atmospheric pressure detection sensor is low, as compared with when the atmospheric pressure is high. That is, when running on an ascending slope road, shift control is performed based on the shift characteristics for running on an ascending slope road in which the predetermined downshift line is moved to a higher engine load side. When the atmospheric pressure on the ascending slope road is low, the amount of oxygen supplied to the engine decreases and the output of the engine decreases. Therefore, the moved downshift line is changed to a lower engine load side, and then the shift control is performed. Accordingly, when running on the ascending slope road where the atmospheric pressure is low, downshifting from a specific shift speed becomes more likely to be performed. Thus, since the shift control is performed based on the shift map in which the predetermined downshift line is set to be on a smaller throttle opening amount side as compared with the ascending slope road shift map when running on the ascending slope road where the atmospheric pressure is low, predetermined downshifting becomes more likely to be performed, and shifting is performed according to the driver's intention.
The third to second downshift line in the ascending slope road shift map is moved to a larger throttle opening amount side on the assumption that the ascending slope road is in a lowland area. Therefore, when running on an ascending slope road in a highland area where driving force of the engine decreases as compared with a lowland area, downshifting to second shift speed is less likely to be performed, and the driving force continues to be insufficient. In order to solve this problem, in the case of the control apparatus disclosed in Japanese Patent Laid-Open Publication No. 2000-88091, the third to second downshift line is moved to a smaller throttle opening amount side as the detected atmospheric pressure is lower, and shift control is performed based on the corrected shift map.
However, in the case of the control apparatus disclosed in the Japanese Patent Laid-Open Publication No. 2000-88091, the shift line in shift control for an ascending slope road is moved, and then the shift line is corrected so as to correspond to an ascending slope road in a highland area. Particularly, in the case of the control apparatus disclosed in the aforementioned patent document, a road surface inclination θ is calculated according to an equation, (generated driving force calculated based on engine torque−acceleration resistance−air resistance−rolling resistance)/vehicle weight. When running on an ascending slope road whose road surface inclination θ is equal to or larger than a threshold value, the shift line is corrected. At this time, the threshold value is a fixed value, and the Japanese Patent Laid-Open Publication No. 2000-88091 does not disclose a technology in which the shift line is changed according to the altitude.
In a highland area, it is necessary to determine whether a road surface on which the vehicle is running has such a large inclination that control for an ascending slope road (hereinafter referred to as “ascending slope control”) needs to be performed, not only in the case of the control apparatus disclosed in the Japanese Patent Laid-Open Publication No. 2000-88091 but also in other cases. Based on this determination, the ascending slope control which changes shift line as disclosed in the Japanese Patent Laid-Open Publication No. 2000-88091, or the ascending slope control which prohibits upshifting is performed. Even in the case where the ascending slope control which prohibits upshifting using a normal shift line is performed, when the determination on an ascending slope road is made based on the road surface inclination calculated according to a running state of the vehicle and a fixed threshold value, there is a possibility that a correct determination cannot be made in a highland area. This is because excess driving force of the engine decreases in a highland area when using a threshold value that is set to be optimal on the assumption that the vehicle runs in a lowland area. As a result, the ascending slope control which prohibits upshifting becomes less likely to be performed in a highland area. Thus, busy shift occurs in which upshifting and downshifting are repeatedly performed, and driveability deteriorates.